Congenital hyperthyroidism is found to occur in 1 : 4 000 – 1 : 40 000 newborn. The disease can be divided in two groups, autoimmune congenital hyperthyroidism and non-autoimmune congenital hyperthyroidism. In the first group the mothers of the children most often have Graves disease, and the infant’s disease is transient, as the antibodies disappears from the child’s circulation in the first 6 months. In the latter group are mutations of the thyroid receptor the cause, and the hyperthyroidism is persistent.Children with congenital hyperthyroidism show more often reduced psycho-motoric skills than healthy children. Thyroid hormone (TH) is found to be important for the development of the cerebellum. Early in development of the nervous system TH balance is assumed to be important because the presence of TH and TH-regulating molecules in nervous tissue. In the postnatal period the migration of neurons in the cerebellum is affected by excess TH in the circulation and brain tissue. To reduce the impact of the disease it is adviced to treat mothers with maternal hyperthyroidism. Pregnant women can have transient hyperthyroidism due to increase in hCG-levels above normal. Altough this may result in fetal hyperthyroidism early in pregnancy, the condition is not associated with negative pregnancy outcome.To study the influence of TH on basic mechanisms such as neuronal proliferation and apoptosis, chicken embryos were injected with increasing doses of thyroxine at day 16 in a pilot study. After 24 hours incubation the chicken embryos was decapiteted. The pilot study shows increased rates of apoptosis and mitosis in IGL in cerebellum of hyperthyroid chicken embryos.